A well-known optical fiber communications network includes a central access switch or optical line termination (OLT), several optical network terminations (ONTs) and one or more optical distribution networks also referred to as passive optical networks (PONs). Such networks typically use time division multiplexing of time slots from the OLT downstream to the ONTs and time division multiple access of time slots from the ONTs upstream to the OLT over the PON. The PON offers one or more optical paths between one OLT and one or more ONTs. The characteristics of the PON are such that each ONT may be at a different distance from the OLT. This distance implies that each ONT""s transmissions are delayed by a fixed but arbitrary amount when compared to the OLT clock. These delays need to be compensated for in order to keep transmissions of multiple ONTs from possibly overlapping and producing data loss.
In systems compatible with ITU Recommendation G.983.1 xe2x80x9cBroadband Optical Access Systems Based on Passive Optical Networksxe2x80x9d (October 1998) (incorporated herein by reference), the OLT controls upstream transmissions from ONTs by sending grants. A grant is a permission to transmit upstream data.
One approach to controlling transmissions on a PON to avoid collisions is referred to as range compensation. Range compensation protocols (e.g., U.S. Pat. No. 5,322,277) operate by first measuring the transmission delay of each ONT at the OLT. The measured transmission delays are then communicated by the OLT to the respective ONTs. Each ONT further delays its transmissions by a fixed amount such that all the ONTs share a common delay. With the ONTs synchronized in this manner, their transmissions can interleave without the risk of collision. In addition, the entire upstream frame is used to carry data and the ONTs can transmit in arbitrary order.
Another approach is called the ascending delay protocol (ADP) which is disclosed in U.S. Pat. No. 5,930,018. The ADP operates by first measuring the transmission delay of each ONT at the OLT. The ONTs are granted bandwidth in order of ascending delay, and the OLT leaves the ending portion of a transmission frame of time slots unassigned. This unassigned portion has a duration equal to the maximum differential delay between the closest and farthest ONTs in distance from the OLT. In the ADP, the ONTs transmit without delay, and because of their ascending ordering, no collisions result.
The range compensation protocol requires communication of ranging delay information to the ONTs. A cost of the ascending delay protocol is the unassigned time required at the end of the frame. For typical systems which can have 20 km logical differential delay, this implies on the order of 200 microseconds of unassigned time. Depending on the frame duration, this unassigned time can become a significant waste of capacity.
There is a need for an approach to controlling transmissions on a PON that eliminates having to communicate ranging delay information to the ONTs while also reducing unassigned time required per transmission frame.
Accordingly, the present invention operates in a communications network which includes an OLT and plural ONTs connected to a common transmission medium (e.g., a passive optical network) and uses a transmission frame structure having time slots for communication. The approach is referred to herein as the ascending delay modulo slot protocol. A method includes measuring round trip transmission delay expressed in time slot duration for each of the ONTs on the transmission medium and ordering the ONTs in ascending order of respective delay modulo time slot duration. A desired reception time at the OLT is scheduled for each of the OLTs according to the ascending order and each ONT is commanded to transmit at a command time that leads the respective desired reception time by a respective delay integral time slot duration.
The scheduling includes leaving a reception interval spare after the last ONT in ascending order. The reception interval comprises one time slot. The present approach, an improvement over the known ascending delay protocol, becomes more efficient as the number of time slots per frame increases. If the number of time slots per frame is N, then the efficiency of the protocol is (Nxe2x88x921)/N.
A communications network according to the present invention includes plural ONTs and an OLT connected to a PON. The OLT includes means for measuring round trip transmission delay expressed in time slot duration for each of the ONTs, means for ordering the ONTs in ascending order of respective delay modulo time slot duration, means for scheduling a desired reception time at the OLT for each of the OLTs according to the ascending order and means for commanding each ONT to transmit at a command time that leads the respective desired reception time by a respective delay integral time slot duration.